Method for gain equalization, and device and system for use in carrying out the method

ABSTRACT

The present invention relates to a method for gain equalization, for example. First, an optical transmission line including an optical amplifier having a gain changing nonlinearly with wavelength is provided (step (a)). Secondly, gain equalization of the optical transmission line is performed so as to obtain a gain changing substantially linearly with wavelength (step (b)). Finally, gain equalization of the optical transmission line is performed so as to obtain a gain remaining substantially unchanged with wavelength (step (c)). According to this method, gain equalization of the optical transmission line is performed so as to obtain a gain remaining substantially unchanged with wavelength after the step (b) of performing gain equalization so as to obtain a gain changing substantially linearly with wavelength. Accordingly, variations in equalization error due to changes in system condition can be easily suppressed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for gain equalization, and adevice and system for use in carrying out the method.

2. Description of the Related Art

In recent years, a manufacturing technique and using technique for alow-loss (e.g., 0.2 dB/km) optical fiber have been established, and anoptical communication system using the optical fiber as a transmissionline has been put to practical use. Further, to compensate for losses inthe optical fiber and thereby allow long-haul transmission, the use ofan optical amplifier for amplifying signal light has been proposed orput to practical use.

An optical amplifier known in the art includes an optical amplifyingmedium to which signal light to be amplified is supplied and means forpumping the optical amplifying medium so that the optical amplifyingmedium provides a gain band including the wavelength of the signallight. For example, an erbium doped fiber amplifier (EDFA) includes anerbium doped fiber (EDF) as the optical amplifying medium and a pumpinglight source for supplying pump light having a predetermined wavelengthto the EDF. By preliminarily setting the wavelength of the pump lightwithin a 0.98 μm band or a 1.48 μm band, a gain band including awavelength band of 1.55 μm can be obtained. Further, another typeoptical amplifier having a semiconductor chip as the optical amplifyingmedium is also known. In this case, the pumping is performed byinjecting an electric current into the semiconductor chip.

As a technique for increasing a transmission capacity by a singleoptical fiber, wavelength division multiplexing (WDM) is known. In asystem adopting WDM, a plurality of optical carriers having differentwavelengths are used. The plural optical carriers are individuallymodulated to thereby obtain a plurality of optical signals, which arewavelength division multiplexed by an optical multiplexer to obtain WDMsignal light, which is output to an optical fiber transmission line. Onthe receiving side, the WDM signal light received is separated intoindividual optical signals by an optical demultiplexer, and transmitteddata is reproduced according to each optical signal. Accordingly, byapplying WDM, the transmission capacity in a single optical fiber can beincreased according to the number of WDM channels.

In the case of incorporating an optical amplifier into a system adoptingWDM, a transmission distance is limited by the wavelength characteristicof gain which is represented by a gain tilt or gain deviation. Forexample, in an EDFA, it is known that a gain tilt is produced atwavelengths in the vicinity of 1.55 μm, and this gain tilt varies withtotal input power of signal light and pump light power to the EDFA.

A gain equalization method is known as measures against the wavelengthcharacteristic of gain of an optical amplifier. This method will bedescribed with reference to FIGS. 1 to 3.

FIG. 1 is a block diagram showing a conventional optical communicationsystem adopting WDM. A plurality of optical signals having differentwavelengths are output from a plurality of optical senders (OS) 2 (#1 to#N), respectively, and next wavelength division multiplexed in anoptical multiplexer 4 to obtain WDM signal light. The WDM signal lightis next output to an optical transmission line 6. The opticaltransmission line 6 is configured by inserting a plurality of opticalamplifiers 8 for compensating for losses and at least one gain equalizer10 in an optical fiber transmission line 7. Each gain equalizer 10 maybe provided by an optical filter. The WDM signal light transmitted bythe optical-transmission line 6 is separated into individual opticalsignals according to wavelengths by an optical demultiplexer 12, andthese optical signals are next supplied to a plurality of opticalreceivers (OR) 14 (#1 to #N), respectively.

Referring to FIG. 2, there is shown an example of the spectrum of theWDM signal light output from the optical multiplexer 4 to the opticaltransmission line 6 in the system shown in FIG. 1. In FIG. 2, thevertical axis represents optical power, and the horizontal axisrepresents wavelength. In this example, the optical senders 2 (#1 to #N)output optical signals having wavelengths (λ₁ to λ_(N)) respectively.When preemphasis is not considered, the optical powers of the opticalsignals in all the channels are equal to each other in general. In thisexample, the band of the WDM signal light is defined by the wavelengthrange of λ₁ to λ_(N) as shown by reference numeral 16.

If each optical amplifier 8 in the system shown in FIG. 1 has awavelength characteristic of gain in the band 16 of the WDM signallight, a gain tilt or gain deviation is accumulated over the length ofthe optical transmission line 6, causing an interchannel deviation insignal power or signal-to-noise ratio (optical SNR). In the gainequalization method, the wavelength characteristic of loss of each gainequalizer 10 is set so as to cancel the wavelength characteristic oftotal gain of the cascaded optical amplifiers 8. This will now bedescribed more specifically with reference to FIG. 3.

In FIG. 3, the broken line shown by reference numeral 18 represents thewavelength characteristic of total gain of the cascaded opticalamplifiers 8, and the solid line shown by reference numeral 20represents the wavelength characteristic of total loss in the gainequalizer(s) 10. In the example shown, the wavelength characteristic oftotal gain is canceled by the wavelength characteristic of total loss inthe band 16 of the WDM signal light, thereby achieving gain equalizationin the whole of the optical transmission line 6.

In the case that an EDFA is used as each optical amplifier 8, thewavelength characteristic of gain of the EDFA is asymmetrical withrespect to a wavelength axis in general. In contrast, the wavelengthcharacteristic of loss of one optical filter usable as an element ofeach gain equalizer 10 is symmetrical with respect to a wavelength axisin general. Accordingly, in the case that each gain equalizer 10includes only one optical filter, the asymmetrical wavelengthcharacteristic of total gain of the cascaded optical amplifiers 8 cannotbe compensated. As the optical filter, a dielectric multilayer filter,etalon filter, Mach-Zehnder filter, etc. are known. These filters can beprecisely manufactured, and the reliability has been ensured.

As the related prior art to compensate for the asymmetrical wavelengthcharacteristic of an optical amplifier, it has been proposed toconfigure a gain equalizer by combining two or more optical filtershaving different wavelength characteristics of loss. With thisconfiguration, the wavelength characteristic of gain can be canceled bythe wavelength characteristic of loss with high accuracy in a given bandof WDM signal light.

Additional information on the gain equalization method is described inReference (1) shown below, and additional information on the combinationof plural optical filters is described in References (2), (3), and (4)shown below.

(1) N. S. Bergano et al., “Wavelength division multiplexing in long-haultransmission systems”, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 14, NO. 6,JUNE 1996, pp1229-1308.

(2) K. Oda et al., “128-channel, 480-km FSK-DD transmission experimentusing 0.98 μm pumped erbium doped fibre amplifiers and a tunable gainequaliser”, ELECTRONICS LETTERS, 9th June 1994, Vol. 30, No. 12,pp982-983.

(3) T. Naito et al., “85-Gb/s WDM transmission experiment over 7931-kmusing gain equalization to compensate for asymmetry in EDFA gaincharacteristics”, First Optoelectronics and Communications Conference(OECC '96) Technical Digest, July 1996, PD1-2.

(4) T. Oguma et al., “Optical gain equalizer for optical fiberamplifier”, Communications Society Conference, IEICE, 1996, B-1093(pp578).

The wavelength characteristic of gain of an optical amplifier changesaccording to operating conditions such as a pumped condition of theoptical amplifier and an input power of signal light. In a submarineoptical repeater system, for example, there is a case that the inputpower to an optical amplifier may change because of an increase inoptical fiber loss due to aging or because of cable patching forrepairing. Such a change in system condition causes a change inoperating conditions of the optical amplifier, resulting in a change inits wavelength characteristic of gain. Further, there is a possibilitythat the wavelength characteristic of gain may deviate from a designvalue because of variations in quality of optical amplifiersmanufactured.

In the conventional gain equalization method using an optical filterhaving a fixed wavelength characteristic of loss, there arises a problemsuch that when the wavelength characteristic of gain of an opticalamplifier changes from a characteristic shown by reference numeral 18 toa characteristic shown by reference numeral 18′ in FIG. 4 because of achange in system condition, the new wavelength characteristic of gain ofthe optical amplifier does not coincide with the wavelengthcharacteristic of loss of the optical filter, causing an equalizationerror. The equalization error varies according to a system condition,and a large amount of variations in the equalization error may cause aninterchannel deviation in signal power or optical SNR or may remarkablydeteriorate a transmission quality in a certain channel.

From this point of view, there has been proposed a method using avariable gain equalizer having a variable wavelength characteristic ofloss. As the variable gain equalizer, an optical device using aMach-Zehnder type band rejection optical filter has been proposed.

However, the conventional variable gain equalizer cannot obtain anarbitrary wavelength characteristic of loss in response to variations inequalization error, so that variations in equalization error due tochanges in system condition cannot be sufficiently suppressed.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodfor gain equalization which can suppress variations in equalizationerror due to changes in system condition.

It is another object of the present invention to provide a novel device(gain equalizer) and system for use in carrying out such a method.

Other objects of the present invention will become apparent from thefollowing description.

In accordance with a first aspect of the present invention, there isprovided a method for gain equalization. First, an optical transmissionline including an optical amplifier having a gain changing nonlinearlywith wavelength is provided(step (a)). Secondly, gain equalization ofthe optical transmission line is performed so as to obtain a gainchanging substantially linearly with wavelength (step (b)). Finally,gain equalization of the optical transmission line is performed so as toobtain a gain remaining substantially unchanged with wavelength (step(c)).

According to this method, gain equalization of the optical transmissionline is performed so as to obtain a gain remaining substantiallyunchanged with wavelength after the step (b) of performing gainequalization so as to obtain a gain changing substantially linearly withwavelength. Accordingly, variations in equalization error due to changesin system condition can be easily suppressed.

Preferably, the step (b) includes a step of using a fixed gain equalizerhaving a fixed wavelength characteristic of gain or loss.

Preferably, the step (c) includes a step of using a variable gainequalizer having a variable wavelength characteristic of gain or loss.In this case, for example, a gain tilt is detected, and the variablegain equalizer is controlled so that the gain tilt detected becomesflat.

In this specification, the wording “gain (or loss) changes linearly withwavelength” means that a linear relation is substantially obtainedbetween gain (or loss) represented by logarithm (e.g., in dB) along avertical axis and wavelength (or frequency) represented by antilogarithmalong a horizontal axis.

In accordance with a second aspect of the present invention, there isprovided a system comprising an optical fiber span, a first gainequalizer, and a second gain equalizer. The optical fiber span includesan in-line optical amplifier. The in-line optical amplifier has a gainchanging nonlinearly with wavelength, for example. The first gainequalizer performs gain equalization of the optical fiber span so as toobtain a gain changing substantially linearly with wavelength. Thesecond gain equalizer performs gain equalization of the optical fiberspan so as to obtain a gain remaining substantially unchanged withwavelength.

In accordance with a third aspect of the present invention, there isprovided a system having an optical fiber span comprising a plurality ofsections each having an in-line optical amplifier. Each of the pluralityof sections comprises a first gain equalizer for substantiallycompensating for a wavelength characteristic of gain in the section anda second gain equalizer for compensating for variations in equalizationerror arising according to the condition of the optical fiber span.

In accordance with a fourth aspect of the present invention, there isprovided a variable gain equalizer applicable to an optical fiber spanhaving a wavelength characteristic of gain. The variable gain equalizercomprises at least two optical switches for switching at least twooptical paths each capable of being a part of the optical fiber span,and at least two optical filters provided on the at least two opticalpaths and having different wavelength characteristics of loss.

In accordance with a fifth aspect of the present invention, there isprovided another method for gain equalization. First, an opticaltransmission line including an optical amplifier is provided. Secondly,a wavelength band of light to be supplied to the optical amplifier islimited so as to obtain a gain changing substantially linearly withwavelength. For example, in the case that WDM signal light is suppliedto the optical amplifier, the wavelength band of the WDM signal light islimited. Finally, gain equalization of the optical transmission line isperformed so as to obtain a gain remaining substantially unchanged withwavelength.

The above and other objects, features and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and appended claims with reference to the attacheddrawings showing some preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a conventional optical communicationsystem adopting WDM (wavelength division multiplexing);

FIG. 2 is a graph showing an example of the spectrum of WDM signal lightin the system shown in FIG. 1;

FIG. 3 is a graph for illustrating a conventional gain equalizationmethod;

FIG. 4 is a graph for illustrating a problem in the conventional gainequalization method;

FIG. 5 is a block diagram showing a first preferred embodiment of theoptical communication system according to the present invention;

FIG. 6 is a block diagram showing a first preferred embodiment of eachsection;

FIG. 7 is a graph for illustrating a change in wavelength characteristicof gain;

FIG. 8 is a graph showing a wavelength characteristic of g(λ)−f(λ);

FIG. 9 is a graph showing a wavelength characteristic of equalizationerror;

FIG. 10 is a graph showing another wavelength characteristic ofequalization error;

FIGS. 11A and 11B are block diagrams showing a first preferredembodiment of a variable gain equalizer applicable to the presentinvention and an improvement of the first preferred embodiment,respectively;

FIG. 12 is a block diagram showing a second preferred embodiment of thevariable gain equalizer;

FIG. 13 is a graph for illustrating a change in wavelengthcharacteristic of gain of the variable gain equalizer shown in FIG. 12;

FIG. 14 is a graph showing an example of the wavelength characteristicof loss of a tunable optical filter;

FIGS. 15A and 15B are block diagrams showing third and fourth preferredembodiments of the variable gain equalizer, respectively;

FIG. 16 is a view showing a positional relation among the members ineach preferred embodiment of the variable gain equalizer shown in FIGS.15A and 15B;

FIG. 17 is a graph showing a wavelength characteristic of transmittance;

FIG. 18 is a graph showing another wavelength characteristic oftransmittance;

FIG. 19 is a block diagram showing an optical spectrum monitorapplicable to the present invention;

FIG. 20 is a graph for illustrating the operation of the opticalspectrum monitor shown in FIG. 19;

FIG. 21 is a block diagram showing a second preferred embodiment of eachsection;

FIG. 22 is a block diagram showing a third preferred embodiment of eachsection;

FIG. 23 is a block diagram showing a second preferred embodiment of theoptical communication system according to the present invention;

FIGS. 24A and 24B are block diagrams showing first and second preferredembodiments of a variable gain equalizing unit applicable to the presentinvention, respectively;

FIG. 25 is a block diagram showing a third preferred embodiment of thevariable gain equalizing unit;

FIG. 26 is a block diagram showing a fourth preferred embodiment of thevariable gain equalizing unit;

FIG. 27 is a graph showing an example of the wavelength characteristicsof loss of two optical filters shown in FIG. 26;

FIGS. 28A and 28B are block diagrams showing essential parts of fifthand sixth preferred embodiments of the variable gain equalizing unit,respectively;

FIG. 29 is a block diagram showing a third preferred embodiment of theoptical communication system according to the present invention;

FIG. 30 is a block diagram showing a fourth preferred embodiment of theoptical communication system according to the present invention; and

FIG. 31 is a graph for illustrating another preferred embodiment of themethod according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Some preferred embodiments of the present invention will now bedescribed in detail.

FIG. 5 is a block diagram showing a first preferred embodiment of theoptical communication system according to the present invention. Thissystem includes an optical transmitting device 22, an optical receivingdevice 24, and an optical fiber span 26 laid between the devices 22 and24. The optical fiber span 26 includes a plurality of sections 28cascaded. The optical transmitting device 22 includes a plurality ofoptical transmitters 2 (#1 to #N) for outputting optical signals havingdifferent wavelengths, and an optical multiplexer 4 for wavelengthdivision multiplexing these optical signals to obtain wavelengthdivision multiplexed signal light (WDM signal light). The WDM signallight thus obtained is supplied to the optical fiber span 26. Theoptical receiving device 24 includes an optical demultiplexer 12 forseparating the WDM signal light from the optical fiber span 26 accordingto wavelengths to obtain optical signals in individual channels, and aplurality of optical receivers 14 (#1 to #N) for receiving these opticalsignals.

FIG. 6 is a block diagram showing a first preferred embodiment of eachsection 28. The section 28 shown in FIG. 6 includes an optical fiber 30,a plurality of optical repeaters 32 inserted in the optical fiber 30, afixed gain equalizer 34 inserted in the optical fiber 30, and a variablegain equalizing unit 36 inserted in the optical fiber 30. Each opticalrepeater 32 includes an in-line optical amplifier 38. As the opticalamplifier 38, an erbium doped fiber amplifier (EDFA) may be adopted. TheEDFA is usable also as a variable gain equalizer to be hereinafterdescribed.

The variable gain equalizing unit 36 includes a variable gain equalizer40 having a variable wavelength characteristic of gain or loss.Preferably, the variable gain equalizing unit 36 is located at the mostdownstream position in the section 28 in respect of a propagationdirection of signal light, so as to facilitate control of the variablegain equalizer 40.

In this preferred embodiment, the fixed gain equalizer 34 is locatedbetween two adjacent ones of the optical repeaters 32. Examples of thefixed gain equalizer 34 include a dielectric multilayer filter, etalonfilter, Mach-Zehnder filter, fiber grating filter, and the combinationthereof.

According to an aspect of the present invention, the fixed gainequalizer 34 performs gain equalization of the section 28 so as toobtain a gain changing substantially linearly with wavelength, and thevariable gain equalizing unit 36 performs gain equalization of thesection 28 so as to obtain a gain remaining substantially unchanged withwavelength.

According to another aspect of the present invention, the fixed gainequalizer 34 substantially compensates for a wavelength characteristicof gain in the section 28, and the variable gain equalizing unit 36compensates for variations in equalization error occurring according tothe condition of the section 28 or the optical fiber span 26 (see FIG.5). The gain in the section 28 may be considered as including loss inthe optical fiber 30.

For example, the above wavelength is limited by a predetermined band. Inthe case that each optical amplifier 38 is a usual EDFA, the abovepredetermined band is definable by a range of about 1540 nm to about1565 nm.

In the preferred embodiment shown in FIG. 6, it is preferable toproperly design the wavelength characteristic of loss of the fixed gainequalizer 34, so as to facilitate coincidence of the equalization errorremaining after gain equalization by the fixed gain equalizer 34 and thewavelength characteristic of gain of the variable gain equalizing unit36. A specific method therefor will now be described.

FIG. 7 is a graph for illustrating a change in the wavelengthcharacteristic of gain in the section 28. It is assumed that thewavelength characteristic of gain(or loss) in the section 28 changesbetween g(λ) [dB] and f(λ) [dB] according to the condition of theoptical fiber span 26 (e.g., increase or decrease in loss in the opticalfiber 30), where g(λ) represents a wavelength characteristic such thatthe gain at a longer wavelength is maximum and the gain at a shorterwavelength is minimum, and f(λ) represents a wavelength characteristicsuch that the gain at a shorter wavelength is maximum and the gain at ashorter wavelength is minimum.

As shown in FIG. 8, the difference between g(λ) and f(λ) shows asubstantially linear relation to wavelength λ. That is, the followingapproximation can be made.

g(λ)−f(λ)≈cλ  (1)

where a wavelength-independent constant term on the right side has beenomitted. The characteristic as expressed by Eq. (1) is not specific tothe system, but it is a characteristic generally obtained in a systemusing a usual EDFA.

The wavelength characteristic of loss(or gain), L(λ) [dB], of the fixedgain equalizer 34 is designed so as to satisfy the following relation.

L(λ)=(g(λ)+f(λ))/2+aλ+b   (2)

In the case that the wavelength characteristic of loss of the fixed gainequalizer 34 is thus designed, equalization errors Δg(λ) and Δf(λ)respectively for g(λ) and f(λ) are given as follows:

Δg(λ)=g(λ)−L(λ)=(g(λ)−f(λ))/2−aλ−b=(c/2−a)λ−b   (3)

Δf(λ)=f(λ)−L(λ)=(f(λ)−g(λ))/2−aλ−b=(−c/2−a)λ−b   (4)

As understood from Eqs. (3) and (4), the equalization error issubstantially linear with respect to wavelength, and the slope of theequalization error depends on the coefficient a. The coefficient a is aquantity determined according to the characteristic of the variable gainequalizing unit 36. The coefficient b is an offset loss of the fixedgain equalizer 34, which is a quantity not directly relating to gaindeviation.

FIGS. 9 and 10 are graphs showing examples of a wavelengthcharacteristic of equalization error. In FIG. 9, a=0 dB/nm and the slopeof the wavelength characteristic of equalization error is changeablebetween a positive value and a negative value. In FIG. 10, a=0.2 dB/nmand the slope of the wavelength characteristic of equalization error ischangeable between zero and a positive value.

In the case that the wavelength characteristic of equalization error issubstantially linear with respect to wavelength as mentioned above, thevariable gain equalizer 40 having a gain or loss changing substantiallylinearly with wavelength can be used as a component of the variable gainequalizing unit 36. That is, by performing control so as to satisfy arelation of Sa=−Se where Sa is the slope of the gain or loss of thevariable gain equalizer 40 and Se is the slope of the equalizationerror, gain tilt in the optical fiber span 26 becomes always flat.

FIG. 11A is a block diagram showing a first preferred embodiment of thevariable gain equalizer 40. In this preferred embodiment, the variablegain equalizer 40 includes an optical amplifier 42 and a variableoptical attenuator 44 for limiting an optical input of the opticalamplifier 42. The attenuation by the variable optical attenuator 44 isadjusted by a control signal supplied to a control terminal 46. Ingeneral, the operating condition of the optical amplifier 42 changeswith its optical input level, and the wavelength characteristic of gainof the optical amplifier 42 accordingly changes. Accordingly, bycontrolling the attenuation of the variable optical attenuator 44according to the control signal, the wavelength characteristic of gainof the optical amplifier 42 can be controlled. As the variable opticalattenuator 44, an optical device capable of electrically controllingattenuation by applying magneto-optic effects may be used.

In the preferred embodiment shown in FIG. 11A, there is a possibilitythat the noise figure (NF) of the optical amplifier 42 may be increased(degraded) by the loss in the variable optical attenuator 44 because theattenuator 44 is provided on the input side of the optical amplifier 42.This possibility can be eliminated by a modification shown in FIG. 11B,wherein the variable gain equalizer 40 is improved by additionallyproviding another optical amplifier 42′ on the input side of theattenuator 44. With this configuration, the loss in the attenuator 44 iscompensated by the optical amplifier 42′, thereby increasing the inputlevel of the optical amplifier 42 to decrease (improve) the noisefigure.

FIG. 12 is a block diagram showing a second preferred embodiment of thevariable gain equalizer 40. In this preferred embodiment, the variablegain equalizer 40 includes an erbium doped fiber (EDF) 48, and a WDMcoupler 50 and a laser diode (LD) 52 in combination for supplying pumplight to the EDF 48. Signal light to be amplified is supplied to a firstend of the EDF 48, and pump light output from the laser diode 52 issupplied through the WDM coupler 50 to a second end of the EDF 48. Whenthe signal light is supplied to the EDF 48 being pumped by the pumplight, the signal light is amplified in the EDF 48 and the amplifiedsignal light is passed through the WDM coupler 50 and an opticalisolator 54 in this order to be output from the variable gain equalizer40.

The wavelength characteristic of gain generated in the EDF 48 is changedby adjusting a drive current for the laser diode 52 according to acontrol signal supplied to a control terminal 56.

By using an EDF codoped with a high concentration of Al as the EDF 48,the wavelength characteristic of gain becomes substantially linear inthe predetermined band defined by the range of about 1540 nm to about1565 nm.

Referring to FIG. 13, there is shown a change in the wavelengthcharacteristic of gain of the variable gain equalizer 40 shown in FIG.12. More specifically, there are shown in FIG. 13 the spectra of outputlight when WDM signal light of four channels having wavelengths of 1548,1551, 1554, and 1557 nm is input with the same input power into the EDF48 being pumped. In FIG. 13, the vertical axis represents output power(dBm), and the horizontal axis represents wavelength (nm).

The spectrum shown by A corresponds to the case where the power of thepump light is relatively high, causing a negative gain tilt in a band ofabout 1.54 μm to about 1.56 μm. That is, the negative gain tilt is again tilt such that the gain decreases with an increase in wavelength,and the derivative of gain (G) with respect to wavelength (λ) isnegative (dG/dλ<0).

The spectrum shown by C corresponds to the case where the power of thepump light is relatively low, causing a positive gain tilt in a band ofabout 1.54 μm to about 1.56 μm. That is, the positive gain tilt is again tilt such that the gain increases with an increase in wavelength,and the derivative of gain with respect to wavelength is positive(dG/dλ>0).

The spectrum shown by B corresponds to the case where the power of thepump light is optimal so that no gain tilt is caused or the gain tiltbecomes flat in a band of about 1.54 μm to about 1.56 μm, and thederivative of gain with respect to wavelength is zero (dG/dλ=0).

Each spectrum has such a shape that four sharp spectra corresponding tothe optical signals in the four channels are superimposed on an ASE(amplified spontaneous emission) spectrum. It is known that thewavelength characteristic of gain of the EDF 48 to a small signal isdependent upon the ASE spectrum.

As the variable gain equalizer 40, a tunable optical filter such as aMach-Zehnder optical filter and an AOTF (acousto-optic tunable filter)may also be used.

FIG. 14 is a graph showing an example of the wavelength characteristicof loss of a tunable optical filter. In this example, the wavelengthcharacteristic of loss is variable in a range between reference numerals58 and 60, that is, a band rejection characteristic is obtained.Accordingly, by using this tunable optical filter in a region where theloss changes substantially linearly with wavelength as shown byreference numeral 62 or 64, the equalization error remaining after gainequalization by the fixed gain equalizer 34 can be compensated.

Referring to FIGS. 15A and 15B, there are shown third and fourthpreferred embodiments of the variable gain equalizer 40, respectively.In each preferred embodiment, a birefringent plate BP and a variableFaraday rotator FR are provided between a first polarizer P1 and asecond polarizer P2. The first polarizer P1 has a transmission axis P1Adetermining the polarization axis of transmitted polarized light, andthe second polarizer P2 has a transmission axis P2A determining thepolarization axis of transmitted polarized light. The birefringent plateBP has optic axes or axis (C1 axis and C2 axis or any one of them)determining a phase difference given between two orthogonal componentsof transmitted polarized light. The variable Faraday rotator FR gives avariable Faraday rotation angle to transmitted polarized light. Theorder of arrangement of the birefringent plate BP and the variableFaraday rotator FR and the relative positional relation between theoptic axis (e.g., C1 axis) and each of the transmission axes P1A and P2Aare set so that the shape of a characteristic curve giving a wavelengthcharacteristic of transmittance changes along the transmittance axisaccording to a change in the Faraday rotation angle.

In the third preferred embodiment shown in FIG. 15A, input light istransmitted through the first polarizer P1, the birefringent plate BP,the variable Faraday rotator FR, and the second polarizer P2 in thisorder along the optical path OP.

In the fourth preferred embodiment shown in FIG. 15B, input light istransmitted through the first polarizer P1, the variable Faraday rotatorFR, the birefringent plate BP, and the second polarizer P2 in this orderalong the optical path OP.

FIG. 16 shows a positional relation among the members in each preferredembodiment of the variable gain equalizer 40 shown in FIGS. 15A and 15B.It is assumed that in the orthogonal three-dimensional coordinate system(X, Y, Z) the Z axis is parallel to the optical path OP, and the Y axisis parallel to the transmission axis P1A of the first polarizer P1.Further, φ, θ, and δ will be defined as follows:

φ: angle formed between the C1 axis of the birefringent plate BP and thetransmission axis P1A (Y axis) of the first polarizer P1. It is assumedthat the angle φ takes a positive sign when rotating clockwise from theY axis toward the C1 axis.

θ: angle formed between the C1 axis of the birefringent plate BP and thetransmission axis P2A of the second polarizer P2. It is assumed that theangle θ takes a positive sign when rotating clockwise from thetransmission axis P2A toward the C1 axis.

δ: angle formed between the transmission axis P1A (Y axis) of the firstpolarizer P1 and the transmission axis P2A of the second polarizer P2.It is assumed that the angle δ takes a positive sign when rotatingclockwise from the Y axis toward the transmission axis P2A.

Accordingly, φ=θ+δ. Further, the Faraday rotation angle α given by theFaraday rotator FR takes a positive sign when rotating counterclockwisefrom the X axis toward the Y axis.

In FIG. 16, the group of an ellipse (including a circle) and straightlines represented by reference symbol PS represents wavelengthdependence of a polarization state at the output of the birefringentplate BP in the case of α=0.

To make the transmitted light intensity of the variable gain equalizer40 have wavelength dependence, the condition that “sin(2φ)sin(2θ) isalways zero” must be avoided. Therefore, in the case of substantiallychanging the angle θ by using the Faraday rotator FR as in the thirdpreferred embodiment shown in FIG. 15A, the angle φ must satisfy φ≠nπ/2(n is an integer). Further, in the case of substantially changing theangle φ by using the Faraday rotator FR as in the fourth preferredembodiment shown in FIG. 15B, the angle θ must satisfy θ≠nπ/2 (n is aninteger).

According to the optical theory, a polarization state of light and anoperation of an optical element acting on its transmitted light can berepresented by a 1×2 matrix known as the Jones vector and a 2×2 matrixknown as the Jones matrix. Further, optical power at each transmissionpoint can be expressed as the sum of the squares of two components ofthe Jones vector. By matrix calculation using the Jones vector and theJones matrix, the transmittance (power transmittance) of the variablegain equalizer 40 can be calculated.

FIG. 17 shows the results of calculation of a wavelength characteristicof transmittance in the third preferred embodiment shown in FIG. 15Aunder the conditions that the angles φ and δ are set to φ=π/4 and δ=0and the Faraday rotation angle α is changed. In FIG. 17, the verticalaxis represents transmittance (dB) and the horizontal axis representsrelative wavelength normalized by FSR. As apparent from FIG. 17, theshape of the characteristic curve giving the wavelength characteristicof transmittance changes along the transmittance axis (the verticalaxis) with a change in the Faraday rotation angle α in the conditionthat the points corresponding to relative wavelengths of 0.25 and −0.25are fixed points.

By changing the Faraday rotation angle α in the range of −δ<α<π/2−δ(range of π/2) in the case of φ=π/4 or in the range of −δ>α>−π/2−δ(range of π/2) in the case of φ=−π/4, all obtainable conditions of thewavelength characteristic of transmittance can be realized.

According to this relation, it is understood that in the case of δ=0,that is, in the case that the transmission axes P1A and P2A are madeparallel to each other, it is sufficient to select either a positivesign or a negative sign for the Faraday rotation angle α to be changed.Accordingly, by setting δ=0, 0<α<π/2 or 0>α>−π/2 is given, so that aFaraday rotator giving a Faraday rotation angle α in only one directioncan be used, thereby simplifying the configuration of the Faradayrotator FR. This effect is similarly exhibited also in the fourthpreferred embodiment shown in FIG. 15B.

Conversely, by using a variable Faraday rotator capable of giving aFaraday rotation angle α in opposite directions and setting δ=φ, thetransmittance becomes constant irrespective of wavelength when α=0. Forexample, in the case that the variable gain equalizer 40 is incorporatedinto a system, there is a case that a constant transmittance ispreferable irrespective of wavelength when control becomes off to resultin α=0. In this case, −π/4<α<π/4 holds, so that the absolute value ofthe Faraday rotation angle α is smaller than π/4. Accordingly, in thecase that a variable Faraday rotator applying a magneto-optic effect isused, it is possible to reduce the power consumption when the Faradayrotation angle α is set to a maximum value. Similar discussions applyalso to the fourth preferred embodiment shown in FIG. 15B, in which itis sufficient to set δ=θ.

The variable gain equalizer 40 having such a characteristic as shown inFIG. 17 has a variable loss tilt. The term of “loss tilt” indicates aslope of a linear characteristic curve giving a wavelengthcharacteristic of transmittance represented by logarithm.

In the case of using the variable gain equalizer 40 having such acharacteristic as shown in FIG. 17, an average of losses in an operatingwavelength band (which will be hereinafter referred to as “averageloss”) can be maintained constant by selecting the operating wavelengthband in the following manner, for example. That is, a center valuebetween adjacent two wavelengths of some wavelengths providing a maximumloss or a minimum loss is selected as a center wavelength in theoperating wavelength band, and the bandwidth of the operating wavelengthband is set smaller than ½ of FSR (free spectral range). FSR representsa spectral period in the wavelength characteristic of transmittance.

FIG. 18 shows an example obtained by selecting a point C giving a centervalue between a point A and a point B each providing a maximum loss or aminimum loss in the graph shown in FIG. 17 as a center wavelength in theoperating wavelength band, and by setting the bandwidth of the operatingwavelength band to ⅕ of FSR. As apparent from FIG. 18, a characteristicwith a variable loss tilt is obtained. Further, as also apparent fromFIG. 18, the average loss does not change irrespective of a change inthe Faraday rotation angle α. In the graph shown in FIG. 18, a perfectlystraight line is shown by a broken line to clearly indicate that eachcharacteristic curve is substantially linear.

FIG. 19 is a block diagram showing the configuration of an opticalspectrum monitor 66 applicable to the present invention. The opticalspectrum monitor 66 can be used as a component of the variable gainequalizing unit 36.

The optical spectrum monitor 66 includes an optical branching circuit 68for branching a light beam from the optical fiber span 26 to obtain abranch beam, a wavelength selecting optical filter 70 for separating thebranch beam into first and second light beams having different bands,and photodetectors (PDs) 72 and 74 for detecting powers of the first andsecond light beams, respectively. Output signals from the photodetectors72 and 74 are supplied to an electrical circuit (or control circuit) 76.

The first light beam has a band including longer-wavelength signals asshown by reference numeral 78 in FIG. 20, and the second light beam hasa band including shorter-wavelength signals as shown by referencenumeral 80 in FIG. 20. Accordingly, the output signal from thephotodetector 72 reflects an optical power P1 in the band 78, and theoutput signal from the photodetector 74 reflects an optical power Ps inthe band 80.

For example, feedback control of the variable gain equalizer 40 can beperformed by using an output signal from the electrical circuit 76 sothat the output signal reflects (P1−Ps). That is, the wavelengthcharacteristic of gain or loss in the variable gain equalizer 40 can becontrolled so that the optical power P1 in the band 78 and the opticalpower Ps in the band 80 are balanced with each other.

In this manner, by detecting a gain tilt in the optical fiber span 26and controlling the variable gain equalizer 40 so that the gain tiltdetected becomes substantially flat, variations in the equalizationerror due to changes in the system condition can be suppressed.

The optical spectrum monitor 66 may be configured instead in accordancewith the method disclosed in Japanese Patent Laid-open No. 9-159526.

FIG. 21 is a block diagram showing a second preferred embodiment of eachsection 28. In the first preferred embodiment shown in FIG. 6, thesingle fixed gain equalizer 34 is inserted in the optical fiber 30. Incontrast therewith, a plurality of fixed gain equalizers 34 are used inthe second preferred embodiment. The plural fixed gain equalizers 34 areprovided in the plural optical repeaters 32, respectively.

In the first preferred embodiment shown in FIG. 6, the fixed gainequalizer 34 substantially compensates the wavelength characteristic ofgain of all the optical amplifiers 38 included in the section 28 and thewavelength characteristic of loss in the optical fiber 30. In contrasttherewith, each fixed gain equalizer 34 in the second preferredembodiment shown in FIG. 21 substantially compensates the wavelengthcharacteristic of gain of the single optical amplifier 38 included inthe corresponding optical repeater 32 and the wavelength characteristicof loss in a transmission line (a part of the optical fiber 30)connected to this optical amplifier 38. Accordingly, each fixed gainequalizer 34 can be easily designed.

FIG. 22 is a block diagram showing a third preferred embodiment of eachsection 28. This preferred embodiment is characterized in that a singlefixed gain equalizer 34 is included in the variable gain equalizing unit36. According to this preferred embodiment, accumulation of thewavelength characteristic of gain of all the optical amplifiers 38included in the section 28 can be easily obtained by measurement, sothat the wavelength characteristic of gain or loss of the fixed gainequalizer 34 in the variable gain equalizing unit 36 located at the mostdownstream position can be easily designed.

Alternatively, the first preferred embodiment of FIG. 6, the secondpreferred embodiment of FIG. 21, and the third preferred embodiment ofFIG. 22 may be combined as required.

In the system shown in FIG. 5, the variable gain equalizing unit 36 isprovided in each section 28 of the optical fiber span 26. Alternatively,the variable gain equalizing unit 36 in the section 28 nearest to theoptical receiving device 24 may be omitted, because the wavelengthcharacteristic of gain remaining in the optical fiber span 26 can becompensated in the optical receiving device 24.

FIG. 23 is a block diagram showing a second preferred embodiment of theoptical communication system according to the present invention. A firstterminal station 82 and a second terminal station 83 are connected tothe opposite ends of the optical fiber span 26. The first and secondterminal stations 82 and 83 include an optical transmitting device 22and an optical receiving device 24 as shown in FIG. 5, respectively. Thefirst terminal station 82 has a supervisory control terminal 84connected to the optical transmitting device 22 for outputting asupervisory signal (SV signal) to the optical fiber span 26. Eachvariable gain equalizing unit 36 has an SV receiver 86 for receiving theSV signal. In each variable gain equalizing unit 36, automatic controland remote control are allowed by the SV signal.

Furthermore, another optical fiber span 26′ is laid between the terminalstations 82 and 83 to allow bidirectional transmission in this preferredembodiment. The terminal station 83 has an optical transmitting device22′ connected to one end of the optical fiber span 26′, and the terminalstation 82 has an optical receiving device 24′ connected to the otherend of the optical fiber span 26′. The optical fiber span 26′ includesan optical fiber 30′, optical repeaters 32′, and variable gainequalizers 40′ respectively corresponding to the optical fiber 30, theoptical repeaters 32, and the variable gain equalizers 40. Effective useof the optical fiber span 26′ will be hereinafter described.

According to this preferred embodiment, the operation of each variablegain equalizer 40 can be switched on and off according to the SV signalreceived by the corresponding SV receiver 86, or each variable gainequalizer 40 can be controlled according to the SV signal received bythe corresponding SV receiver 86.

The SV signal may be superimposed on a main signal to be transmittedfrom the optical transmitting device 22 to the optical receiving device24, or may be transmitted by using an optical signal in a dedicatedchannel of WDM signal light to be transmitted from the opticaltransmitting device 22 to the optical receiving device 24.

FIGS. 24A and 24B are block diagrams showing first and second preferredembodiments of the variable gain equalizing unit 36, respectively.

The preferred embodiment shown in FIG. 24A is applicable to a system inwhich the SV signal is transmitted by using an optical signal in adedicated channel of WDM signal light, for example. The optical signalin the dedicated channel is extracted by a WDM coupler 88, and the SVsignal is regenerated by the SV receiver 86 according to the extractedoptical signal. The optical spectrum monitor 66 shown in FIG. 19, forexample, is provided downstream of the variable gain equalizer 40.Accordingly, feedback control of the variable gain equalizer 40 by thecontrol circuit 76 can be switched on and off according to the SV signalobtained in the SV receiver 86. Further, the characteristic of thevariable gain equalizer 40 can also be forcibly set after the feedbackcontrol. These functions are necessary in system adjustment on theoptical fiber span 26, for example.

The preferred embodiment shown in FIG. 24B is applicable to a system inwhich the SV signal is superimposed on the main signal. As mentionedabove, the control circuit 76 controls the variable gain equalizer 40 sothat the difference between output signals from the photodetectors 72and 74 becomes zero or constant. In this preferred embodiment, the SVreceiver 86 regenerates the SV signal according to the sum of outputsignals from the photodetectors 72 and 74. Then, the feedback control ofthe variable gain equalizer 40 by the control circuit 76 is switched onand off according to the SV signal obtained in the SV receiver 86.

In the preferred embodiment shown in FIG. 24A, the SV receiver 86requires a photodetector (not shown) to regenerate the SV signal. Bycontrast, the preferred embodiment shown in FIG. 24B does not requiresuch a photodetector and the WDM coupler 88.

FIG. 25 is a block diagram showing a third preferred embodiment of thevariable gain equalizing unit 36. In this preferred embodiment, thecontrol circuit 76 directly controls the wavelength characteristic ofgain or loss of the variable gain equalizer 40 according to the SVsignal obtained in the SV receiver 86. The control of the wavelengthcharacteristic of the variable gain equalizer 40 may be performedaccording to an optical spectrum obtained on the receiving side, forexample. According to this preferred embodiment, the optical spectrummonitor 66 is not required, so that the configuration of the variablegain equalizing unit 36 can be simplified.

In modification, the preferred embodiment shown in FIG. 24A or 24B andthe preferred embodiment shown in FIG. 25 may be combined to carry outthe present invention. Such combination may allow selective control suchthat feedback control is performed by the preferred embodiment shown inFIG. 24A or 24B during normal operation, while forcible control isperformed according to the external SV signal by the preferredembodiment shown in FIG. 25 in case of abnormality.

FIG. 26 is a block diagram showing a fourth preferred embodiment of thevariable gain equalizing unit 36. In this preferred embodiment, thevariable gain equalizer 40 includes two optical switches 90 (#1 and #2)for switching two optical paths each capable of being a part of theoptical fiber span 26, and two optical filters 92 (#1 and #2) providedon the two optical paths and having different wavelength characteristicsof loss.

More specifically, the optical switch 90 (#1) is a 1×2 optical switch,and the optical switch 90 (#2) is a 2×1 optical switch. An input port ofthe optical switch 90 (#1) is located on the input side. The opticalfilters 92 (#1 and #2) are located between two output ports of theoptical switch 90 (#1) and two input ports of the optical switch 90(#2). An output port of the optical switch 90 (#2) is located on theoutput side. The control circuit 76 controls the optical switches 90 (#1and #2) according to the SV signal obtained in the SV receiver 86,thereby selecting any one of the two optical paths. As each of theoptical switches 90 (#1 and #2), an optical switch using a magneto-opticeffect may be used.

Referring to FIG. 27, there is shown an example of the wavelengthcharacteristics of loss of the optical filters 92 (#1 and #2) shown inFIG. 26. In this example, one of the optical filters 92 (#1 and #2) hasa wavelength characteristic of loss having a positive slope as shown byreference numeral 94, and the other has a wavelength characteristic ofloss having a negative slope as shown by reference numeral 96.

Also according to the preferred embodiment shown in FIG. 26, thewavelength characteristic of equalization error remaining after gainequalization by the fixed gain equalizers 34 can be compensated tothereby flatten the gain tilt in the optical fiber span 26.

FIGS. 28A and 28B are block diagrams showing essential parts of fifthand sixth preferred embodiments of the variable gain equalizing unit 36,respectively. That is, the configurations of variable gain equalizers 40in the fifth and sixth preferred embodiments are shown.

In contrast with the fourth preferred embodiment shown in FIG. 26, thepreferred embodiment shown in FIG. 28A is characterized in that the twooptical paths are expanded to N optical paths (N is an integer greaterthan 2). That is, optical filters 92 (#1 to #N) are provided in parallelbetween a 1×N optical switches 90 (#1)′ and an N×1 optical switch 90(#2)′. The optical filters 92 (#1 to #N) have different wavelengthcharacteristics of loss. Accordingly, fine compensation of theequalization error can be made in comparison with the fourth preferredembodiment shown in FIG. 26.

In contrast with the fourth preferred embodiment shown in FIG. 26, thepreferred embodiment shown in FIG. 28B is characterized in that a 2×2optical switch 90 (#3) and two optical filters 92 (#3 and #4) areadditionally provided between the optical filters 92 (#1 and #2) and theoptical switch 90 (#2). According to this preferred embodiment, any oneof a cascaded condition of the optical filters 92 (#1 and #3), acascaded condition of the optical filters 92 (#1 and #4), a cascadedcondition of the optical filters 92 (#2 and #3), and a cascadedcondition of the optical filters 92 (#2 and #4) can be selected.Accordingly, fine compensation of the equalization error can be madelike the fifth preferred embodiment shown in FIG. 28A.

FIG. 29 is a block diagram showing a third preferred embodiment of theoptical communication system according to the present invention. In thispreferred embodiment, an optical branching circuit 98 for obtaining abranch beam from the WDM signal light transmitted through the opticalfiber span 26 and an optical spectrum monitor or optical spectrumanalyzer 100 for detecting a gain tilt in the optical fiber span 26according to the branch beam are provided to detect the gain tilt in thesecond terminal station 83. Information on the gain tilt detected is fedfrom the optical spectrum monitor 100 to a supervisory control terminal84′, which in turn operates the optical transmitting device 22′, therebytransmitting an SV signal including the information on the detected gaintilt through the optical fiber span 26′ to the first terminal station82. In the first terminal station 82, the SV signal received isregenerated by the optical transmitting device 24′, and the regeneratedSV signal is supplied to the supervisory control terminal 84.

Accordingly, feedback control such that the gain tilt detected in thesecond terminal station 83 becomes flat can be performed in eachvariable gain equalizing unit 36. That is, the SV signal transmittedfrom the first terminal station 82 to each variable gain equalizing unit36 is used as a control signal to control each variable gain equalizer40 according to the control signal.

Also according to this preferred embodiment, each variable gainequalizer 40 is controlled in the corresponding variable gain equalizingunit 36 according to the control signal, thereby flattening the gaintilt in the optical fiber span 26.

FIG. 30 is a block diagram showing a fourth preferred embodiment of theoptical communication system according to the present invention. Likethe third preferred embodiment shown in FIG. 29, the gain tilt in theoptical fiber span 26 is detected in the second terminal station 83. Bycontrast, the fourth preferred embodiment is characterized in that theinformation on the gain tilt detected is transmitted through the opticalfiber span 26′ directly to each variable gain equalizing unit 36. Morespecifically, each variable gain equalizing unit 36 includes an SVreceiver 86′ connected to the optical fiber span 26′. The SV receiver86′ receives an SV signal transmitted from the second terminal station83 to each variable gain equalizing unit 36 as a control signal.

Accordingly, each variable gain equalizer 40 is controlled according tothe control signal received by the corresponding SV receiver 86′,thereby flattening the gain tilt in the optical fiber span 26.

In modification, the first terminal station 82 may be configuredsimilarly to the second terminal station 83. In this case, the SVreceiver 86 provided in each variable gain equalizing unit 36 is usedfor the optical fiber span 26 to control each variable gain equalizer40′, thereby flattening a gain tilt in the optical fiber span 26′.

FIG. 31 is a graph for illustrating another preferred embodiment of themethod according to the present invention. While the fixed gainequalizer 34 is used to obtain a gain changing substantially linearlywith wavelength in each preferred embodiment mentioned above, the fixedgain equalizer 34 may be omitted in carrying out the present invention.For example, in the case that the wavelength characteristic of gain ofeach optical amplifier 38 is changeable between a characteristic shownby reference numeral 102 and a characteristic shown by reference numeral104 in FIG. 31, a gain changing substantially linearly with wavelengthis always obtained in a band shown by reference numeral 106.Accordingly, by limiting the wavelength band of light to be supplied toeach optical amplifier 38 to the band 106, the fixed gain equalizer 34can be omitted.

In the case that each optical amplifier 38 is an EDFA, the wavelengthband of signal light to be amplified by the EDFA is limited to a rangeof about 1540 nm to about 1565 nm. Such limitation of the wavelengthband can be achieved by properly setting or controlling the wavelengthsof optical signals to be output from the optical transmitters 2 (#1 to#N) shown in FIG. 5, for example.

The present invention is not limited to the details of the abovedescribed preferred embodiments. The scope of the invention is definedby the appended claims and all changes and modifications as fall withinthe equivalence of the scope of the claims are therefore to be embracedby the invention.

What is claimed is:
 1. A method for gain equalization, comprising thesteps of: (a) providing an optical transmission line including anoptical amplifier having a gain changing nonlinearly with wavelength;(b) performing gain equalization of said optical transmission line so asto obtain a gain changing substantially linearly with wavelength; and(c) performing gain equalization of said optical transmission line so asto obtain a gain remaining substantially unchanged with wavelength aftersaid step (b).
 2. A method according to claim 1, wherein said step (b)includes a step of using a fixed gain equalizer having a fixedwavelength characteristic of gain or loss.
 3. A method according toclaim 1, wherein said step (c) includes a step of using a variable gainequalizer having a variable wavelength characteristic of gain or loss.4. A method according to claim 3, wherein said step (c) further includesa step of detecting a gain tilt and a step of controlling said variablegain equalizer so that said gain tilt detected becomes flat.
 5. A methodaccording to claim 1, wherein said wavelength is limited by apredetermined band.
 6. A method according to claim 5, wherein: saidoptical amplifier comprises an erbium doped fiber amplifier; and saidpredetermined band is defined by a range of about 1540 nm to about 1565nm.
 7. A system comprising: an optical fiber span including an in-lineoptical amplifier; a first gain equalizer performing gain equalizationof said optical fiber span so as to obtain a gain changing substantiallylinearly with wavelength; and a second gain equalizer performing gainequalization of said optical fiber span so as to obtain a gain remainingsubstantially unchanged with wavelength.
 8. A system according to claim7, wherein: said first gain equalizer comprises a fixed gain equalizerhaving a fixed wavelength characteristic of gain or loss; and saidsecond gain equalizer comprises a variable gain equalizer having avariable wavelength characteristic of gain or loss, means for detectinga gain tilt in said optical fiber span, and means for controlling saidvariable gain equalizer so that said gain tilt detected becomessubstantially flat.
 9. A system according to claim 7, wherein saidwavelength is limited by a predetermined band.
 10. A system according toclaim 9, wherein: said in-line optical amplifier comprises an erbiumdoped fiber amplifier; and said predetermined band is defined by a rangeof about 1540 nm to about 1565 nm.
 11. A system according to claim 7,wherein when a wavelength characteristic of gain(or loss) of saidoptical fiber span changes between g(λ) [dB] and f(λ) [dB], a wavelengthcharacteristic of loss(or gain), L(λ) [dB], of said first gain equalizeris substantially given by: L(λ)=(g(λ)+f(λ))/2+aλ+b where a and b areconstants.
 12. A system according to claim 11, wherein said second gainequalizer has a gain or loss changing substantially linearly withwavelength.
 13. A system according to claim 7, wherein said second gainequalizer comprises an optical amplifier and a variable opticalattenuator for limiting an optical input of said optical amplifier. 14.A system according to claim 7, wherein: said second gain equalizercomprises an erbium doped fiber amplifier and a pumping source forsupplying pump light to said erbium doped fiber; a wavelengthcharacteristic of gain generated in said erbium doped fiber beingchanged according to a power of said pump light.
 15. A system accordingto claim 7, wherein: said second gain equalizer comprises a variablegain equalizer having a variable wavelength characteristic of gain orloss, a spectrum monitor for detecting a gain tilt in said optical fiberspan, and means for controlling said variable gain equalizer so thatsaid gain tilt detected becomes flat; said spectrum monitor comprisingan optical filter for extracting first and second light beams havingdifferent bands, and means for detecting powers of said first and secondlight beams.
 16. A system according to claim 7, wherein said second gainequalizer comprises at least two optical switches for switching at leasttwo optical paths, and at least two optical filters provided on said atleast two optical paths and having different wavelength characteristicsof loss.
 17. A system according to claim 7, further comprising first andsecond terminal stations connected to opposite ends of said opticalfiber span.
 18. A system according to claim 17, wherein: said firstterminal station includes means for outputting a supervisory signal tosaid optical fiber span; and said second gain equalizer includes avariable gain equalizer having a variable wavelength characteristic ofgain or loss and means for receiving said supervisory signal.
 19. Asystem according to claim 18, wherein said second gain equalizer furtherincludes means for switching on and off an operation of said variablegain equalizer according to said supervisory signal received.
 20. Asystem according to claim 18, wherein said second gain equalizer furtherincludes means for controlling said variable gain equalizer according tosaid supervisory signal received.
 21. A system according to claim 18,wherein: said second terminal station includes means for detecting again tilt in said optical fiber span and means for feeding informationon said gain tilt detected to said first terminal station; said firstterminal station further includes means for generating said supervisorysignal according to said information; and said second gain equalizerfurther includes means for controlling said variable gain equalizeraccording to said supervisory signal received so that said gain tiltbecomes flat.
 22. A system according to claim 17, wherein: said secondterminal station includes means for detecting a gain tilt in saidoptical fiber span and means for feeding information on said gain tiltdetected to said second gain equalizer; and said second gain equalizerincludes a variable gain equalizer having a variable wavelengthcharacteristic of gain or loss, means for receiving said information,and means for controlling said variable gain equalizer according to saidinformation received so that said gain tilt becomes flat.
 23. A systemaccording to claim 7, wherein: said second gain equalizer comprisesfirst and second polarizers, a birefringent element, and a variableFaraday rotator; said birefringent element and said variable Faradayrotator being provided between said first and second polarizers.
 24. Avariable gain equalizer applicable to an optical fiber span having awavelength characteristic of gain or loss, comprising: at least twooptical switches switching at least two optical paths each capable ofbeing a part of said optical fiber span; and at least two opticalfilters provided on said at least two optical paths and having differentwavelength characteristics of loss.
 25. A system having an optical fiberspan comprising a plurality of sections each having an in-line opticalamplifier, each of said plurality of sections comprising: a first gainequalizer substantially compensating for a wavelength characteristic ofgain or loss in said section; and a second gain equalizer compensatingfor variations in equalization error arising according to a condition ofsaid optical fiber span, wherein when the wavelength characteristic ofgain (or loss) in said section changes between g(λ) [dB] and f(λ) [dB],a wavelength characteristic of loss (or gain), L(λ) [dB], of said firstgain equalizer is substantially given by: L(λ)=(g(λ)+f(λ))/2+aλ+b wherea and b are constants.
 26. A system according to claim 25, wherein saidsecond gain equalizer is located at a most downstream position in saidsection.
 27. A system according to claim 25, wherein said second gainequalizer has a gain or loss changing substantially linearly withwavelength.
 28. A system according to claim 25, wherein: said first gainequalizer comprises a fixed gain equalizer having a fixed wavelengthcharacteristic of gain or loss; and said second gain equalizer comprisesa variable gain equalizer having a variable wavelength characteristic ofgain or loss, means for detecting a gain tilt in said section, and meansfor controlling said variable gain equalizer so that said gain tiltdetected becomes substantially flat.
 29. A system according to claim 28,wherein: said variable gain equalizer comprises first and secondpolarizers, a birefringent element, and a variable Faraday rotator; saidbirefringent element and said variable Faraday rotator being providedbetween said first and second polarizers.
 30. A method for gainequalization, comprising the steps of: (a) providing an opticaltransmission line including an optical amplifier; (b) limiting awavelength band of light to be supplied to said optical amplifier so asto obtain a gain changing substantially linearly with wavelength; and(c) performing gain equalization of said optical transmission line so asto obtain a gain remaining substantially unchanged with wavelength aftersaid step (b).
 31. A method of gain equalization, comprising: providingan optical transmission line including an optical amplifier having again changing nonlinearly with wavelength; and equalizing gain of theoptical transmission line to obtain a gain changing substantiallylinearly with wavelength before equalizing gain of the opticaltransmission line to obtain a gain remaining substantially unchangedwith wavelength.
 32. A method of gain equalization, comprising:providing an optical transmission line including an optical amplifier;and limiting a wavelength band of light supplied to the opticalamplifier to obtain a gain changing substantially linearly withwavelength before equalizing gain of the optical transmission line toobtain a gain remaining substantially unchanged with wavelength.